Hypervisor Timers

The Arm generic timers include support for timer virtualization. Generic timers provide a counter that measures (in real-time) the passing of time, and a timer for each CPU. The CPU timers are programmed to raise an interrupt to the CPU after a certain amount of time has passed, as per the counter.

Timers are likely to be used by both hypervisors and guest operating systems. However, to provide isolation and retain control, the timers used by the hypervisor cannot be directly configured and manipulated by guest operating systems. Refer to Timers and Counters for further details.

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Reasons Hypervisors need Generic Timing Counters

Time Isolation Between VMs: In a virtualized environment, different VMs may run on the same physical hardware. Each VM might have different time requirements or be running different operating systems and applications. Generic timing counters help the hypervisor provide an isolated timing context to each VM, ensuring that the timing needs of one VM do not interfere with those of another.

Synchronization and Time Accuracy: Hypervisors use timing counters to maintain a consistent time across all VMs. This is crucial for many applications, especially those that are time-sensitive such as database transactions, network communications, or any system that relies on time-stamping or scheduled tasks.

Efficiency in Time Management: By using generic timing counters, a hypervisor can efficiently manage time without the overhead of emulating specific hardware timers for each virtual machine. This simplifies the architecture of the hypervisor and reduces the computational overhead associated with time management.

Handling Time Drift: In a virtual environment, VMs might experience 'time drift' where the clock inside the VM loses sync with the real-world clock. Generic timing counters allow the hypervisor to adjust and synchronize the clocks inside VMs regularly, thus mitigating time drift issues.

Performance Optimization: Timers are used not just for maintaining the system clock but also for scheduling tasks within the hypervisor and its VMs. Efficient use of generic timing counters can optimize these operations, improving overall system performance and responsiveness.